1. Field of the Invention
The present invention relates to a perpendicular magnetic recording system using a perpendicular magnetic recording medium (double layered perpendicular magnetic recording medium) having a soft magnetic underlayer.
2. Description of the Related Prior Art
In perpendicular magnetic recording systems using a double layered perpendicular magnetic recording medium, magnetic flux extending from a main pole of a recording head follows a magnetic path that runs through a soft magnetic underlayer of the double layered perpendicular recording medium to enter an auxiliary pole of a magnetic head and then returns to the main pole. Conventional perpendicular magnetic recording media have adopted soft magnetic underlayers of greater design thicknesses so as to avoid magnetic saturation of the soft magnetic underlayers.
FIG. 4 is a diagram explaining the cross section of a path through which magnetic flux flows on a conventional model. As shown in FIG. 4, in order to prevent a soft magnetic underlayer 6 of a perpendicular magnetic recording medium from being saturated by the magnetic flux from a main pole 1 of a magnetic head, it is considered that the limit of the prevention is determined by both the area of the cross section 11 of the magnetic flux inside the soft magnetic underlayer, through which the magnetic flux extending from a top face 10 of the main pole passes, and the saturation flux density of the soft magnetic underlayer 6, and it is necessary to satisfy the following relational expression:Tww×Tb1×BS2>Tww×Tm×BS1,i.e.,Tb1×BS2>Tm×BS1,  (1)wherein Tww is the track width of the main pole 1, BS1 the saturation flux density of the main pole 1, Tm the thickness of the main pole 1, BS2 the saturation flux density of the soft magnetic underlayer 6, and Tb1 the thickness of the soft magnetic underlayer. In view of this, perpendicular magnetic recording media have been provided with a thick soft magnetic underlayer.
For example, Japanese Patent Laid-Open Publication No. Hei 10-283624 describes a double layered perpendicular magnetic recording medium having a soft magnetic underlayer of 600 nm in thickness.
In conventional ideas, for example, the saturation flux density Bs of a main pole of 1.6 T, the thickness Tm of the same of 0.5 μm, and the saturation flux density BS of a soft magnetic underlayer of 1.2 T combine to require, according to the expression (1), the thickness Tb1 of the soft magnetic underlayer as great as 0.67 μm or more. This is no less than ten times the thickness of the magnetic recording layers of current in-plane magnetic recording media which is no greater than several tens of nanometers. Given here that the growth rates are nearly equal, the growth time becomes more than ten times, causing a drop in production efficiency and a rise in cost. Moreover, the consumption of the target used in the sputtering also increases for a cost increase. Besides, greater thicknesses deteriorate surface roughness because of inhomogeneous grain growth. This causes a problem since high-density magnetic recording media require low surface roughness for the sake of reducing head-medium spacing. Accordingly, conventional double layered perpendicular magnetic recording media were disadvantageous as compared with in-plane magnetic recording media and single layered perpendicular magnetic recording media.
To avoid the saturation of a soft magnetic underlayer without thickening the soft magnetic underlayer, it is necessary to thin the main pole of the magnetic head or raise the saturation magnetization (Bs) of the soft magnetic underlayer significantly. Nevertheless, when the recording layer of the medium has a relatively high coercivity (Hc), the intensity of write magnetic field must be increased, and the thinning of the main pole produces a problem of main pole saturation.
Furthermore, there are other problems including that no material has been found which can increase the saturation magnetization of soft magnetic underlayers considerably.